Colorants are typically chemical-based dyes that generate colors based on selective absorption of radiation through electronic transitions. Specific chemical groups have disparate electronic transitions, so the production of particular colors introduces chemical reactivity, leading to chemical complexity. Unlike such chemical colorants, physical colorants produce color through physical effects, such as, for example, diffraction, thin film interference, photonic crystal interaction, or plasmon resonance. In addition to producing color, physical colorants can also produce effects that chemical dyes cannot, such as, for example, pearlescence, metallic sheens, infrared blocking, viewing angle dependent color, temperature dependent color, and so forth.
One physical effect capable of modulating color is the so-called Christiansen effect, first described by Christian Christiansen in 1884, using differences in dispersion curves between various materials. A dispersion curve for a material characterizes the frequency dependence of the refractive index of the material. The Christiansen effect may be summarized as monochromatic (or narrow band) transparency effect produced in multiphasic materials when the multiple phases have intersecting dispersion curves. Christiansen discovered that a mass of glass particles immersed in a liquid totally transmitted the light wavelength at which the liquid and the glass particles had the same refractive index. All other wavelengths of the spectrum are reflected, scattered, or refracted at the many interfaces between the liquid and glass particles. It was also observed that the wavelength of transmitted light can be changed by changing the type of glass or changing the temperature of the liquid. The basic theory of the Christiansen effect can be found in the work by Raman (Raman, C. V., “The theory of the Christiansen Experiment”, Proceedings of Indian Academy of Sciences; 1949, A29: 381-390) and Clarke (Clarke, R. H., “A theory of the Christiansen filter”, Applied Optics; 1968, 7:861-868).